Mashhad Imam Yahya ibn al-Qasim was built in 1239 by Badr al-Din Lu'lu', the Atabeg ruler of Mosul who was sympathetic to Shi'ism. The mausoleum is dedicated to a descendant of Ali ibn Abi Talib through his son Hasan called Yahya ibn al-Qasim, as is stated on a wooden cenotaph that was commissioned along with the building and inscribed in Arabic.1 

The mausoleum was situated on the Tigris riverbank, in the vicinity of the citadel, some 200 meters south of the fortification known as Bash Tabiya. Because of erosion and its proximity to the river, the east façade was covered by two heavy buttresses that prevent the structure from falling into the Tigris.

The plan of the building consisted of a square base made of baked bricks topped by a pyramidal roof raised on an octagonal drum, with an inner muquarnas dome. The muqarnas was based on a cross-shaped plan. The entrance was on the north side of the cube and at one point was covered with a porch. This entrance led onto the tomb chamber. Its plan is similar to the roughly contemporary Shrine of Imam 'Awn al-Din, also constructed by Badr al-Din Lu'lu' in Mosul. Like Imam 'Awn al-Din, Imam Yahya ibn al-Qasim has a mihrab situated at the southwest corner of the tomb because the building is not oriented towards Mecca. It is made of two plaques that join at the corner (one on the south and one on the west wall) and features a hanging lamp within an arch framed by an inscription band.

Both interior and exterior of the building were enriched with architectural decoration. The main portal on the north facade was flanked by two decorative niches filled with geometric ornament made of brick and glazed tile. Another niche-shaped panel with geometric decoration surmounted the entrance portal. The niches flanking the door contained turquoise bricks forming interlocking stars and octagons, and had kufic inscriptive panels below the crowning arch. There were similar niches on the west and south facades, placed on either side of a window. 

The interior walls were decorated with brickwork and terracotta panels with geometric ornament arranged symmetrically. At the dado level, an inscription band and vegetal frieze ran around the length of the square chamber. The frieze is comprised of flowers and leaves carved in high relief. The inscription band was created by carving out the letters and inlaying them with a white gypsum paste. There is another inscription band below the dome.

Jones, Christopher. "August in Iraq: More Destruction, Humanitarian Catastrophes." Gates of Nineveh blog, August 14, 2014. Accessed June 30, 2015. https://gatesofnineveh.wordpress.com/2014/08/14/august-in-iraq-more-destruction-humanitarian-catastrophes/.